As known in the art, high selectivity catalysts (HSCs) for the epoxidation of ethylene refer to those catalysts that possess selectivity values higher than high activity catalysts (HACs) used for the same purpose. Both types of catalysts include silver as the active catalytic component on a refractory support (i.e., carrier). Typically, one or more promoters are included in the catalyst to improve or adjust properties of the catalyst, such as selectivity.
Generally, HSCs achieve the higher selectivity (typically, in excess of 87 mole % or above) by incorporation of rhenium as a promoter. Typically, one or more additional promoters selected from alkali metals (e.g., cesium), alkaline earth metals, transition metals (e.g., tungsten compounds), and main group metals (e.g., sulfur and/or halide compounds) are also included.
There are also ethylene epoxidation catalysts that may not possess the selectivity values typically associated with HSCs, though the selectivity values are improved over HACs. These types of catalysts can also be considered within the class of HSCs, or alternatively, they can be considered to belong to a separate class, e.g., “medium selectivity catalysts” or “MSCs.” These types of catalysts typically exhibit selectivities of at least 83 mole % and up to 87 mole %.
In contrast to HSCs and MSCs, the HACs are ethylene epoxidation catalysts that generally do not include rhenium, and for this reason, do not provide the selectivity values of HSCs or MSCs. Typically, HACs include cesium (Cs) as the only promoter.
There has long been an effort to improve the activity and selectivity of ethylene oxidation catalysts. Much of these efforts focus on the compositional and physical characteristics of the carrier (typically alumina), and more particularly, in modifications to the surface area or pore size distribution of the carrier. See, for example, U.S. Pat. Nos. 4,226,782, 4,242,235, 5,266,548, 5,380,697, 5,395,812, 5,597,773, 5,831,037 and 6,831,037 as well as in U.S. Patent Application Publication Nos. 2004/0110973 A1 and 2005/0096219 A1.
Although a higher surface area of the carrier is known to improve catalyst activity, a higher surface area is typically achieved by a concomitant increase in the pore volume contribution of smaller pores (i.e., generally, of or less than 1 micron in size). In turn, the increased amount of smaller pores has a negative effect on the maximum achievable selectivity of the catalyst. Likewise, attempts to improve the selectivity by lowering the volume contribution of smaller pores has the effect of decreasing the surface area of the catalyst, thereby resulting in a decline in the catalyst activity. Thus, there continues to be a long unsolved problem encountered in the art of ethylene oxide catalysts in which improving the activity of the catalyst has a negative impact on the selectivity of the catalyst, and likewise, improving the selectivity has a negative impact on the activity.
Accordingly, there remains a need in the art for improving the catalyst activity while not negatively impacting, or even simultaneously improving, the selectivity of the catalyst. There would be a particular benefit in achieving this by means which are readily integratable into existing process designs, and which are facile and cost effective.